The present invention relates to a method for improving the color and the stability of the color of carboxylic acid or a mixture of the same.
The color and the stability of the color of carboxylic acids, mainly fatty acids, have commonly been improved by using a bleaching-earth treatment, in which bleaching earth is added at 1-20 % to fatty acid and filtered off after mixing. Such a treatment is difficult on an industrial scale, since it requires a mixing tank and a filter. Furthermore, the material being treated must be fluid at relatively low temperatures, i.e. at below 100.degree. C., in order that the treatment should not decompose the carboxylic acids. In addition, some of the product remains in the bleaching earth after the filtration, which means losses of material.
Because of the drawbacks of the above-mentioned method, endeavors have been made to find other methods. U.S. Pat. No. 3,551,404 describes a method in which pine resin is heated at 200-300.degree. C. for 1-20 hours and is thereafter distilled. The color and the odor are thereby improved.
U.S. Pat. No. 3,377,333 describes a method in which phenol sulfides are added to crude tall oil in connection with vacuum distillation. However, sulfur compounds have a disproportioning effect on resin acids and an isomerizing effect on fatty acids, a fact which may be disadvantageous in distillation and in the intended use of the final products.
U.S. Pat. Nos. 4,126,604 and 4,222,933 describe methods in which zinc and iodine, and respectively zinc and boric acid, are added to fatty acids and to resin acids. They have a disadvantage in that it is necessary to add two different substances to the process and that iodine and acids in general have a decarboxylating effect on resin acids and fatty acids.